1. Field of the Invention
The present invention relates generally to sealing devices, and more particularly to gaskets such as gaskets for use in gasoline and diesel engines, compressors, oil coolers, and other machinery.
2. Description of the Related Art
Gaskets have long been used to seal interfaces between components in a wide variety of machines and especially in gasoline and diesel engines. For example, head gaskets seal between the heads of an engine and the engine block, oil pan gaskets seal the interface between the oil pan and the block, and water pump gaskets seal around the ports of a water pump where the water pump is attached to the engine block. Most gaskets are specifically designed for their particular intended use. For instance, head gaskets are designed to seal against the high pressures and temperatures and the generally caustic environment within the cylinders of an engine. On the other hand, water pump gaskets must seal against leakage of coolant, which may consist of a water and anti-freeze mixture that is heated and under pressure. Many if not most automotive gaskets traditionally have been made of a compressible fibrous gasket sheet material that is die-cut to the required gasket shape.
In general, two key performance characteristics required of most compressible gaskets include compression failure resistance and sealability. Compression failure resistance refers to the ability of a gasket to withstand high compression forces when clamped between two flange surfaces without crushing, deforming, or yielding to the point that the mechanical properties of the gasket material and ultimately the seal provided by the gasket are compromised. Sealability refers to a gasket's ability to resist or prevent leakage of fluid both between the gasket faces and the flanges between which the gasket is clamped (hereinafter referred to as interfacial leakage) and through the gasket material itself (hereinafter referred to as intersticial leakage).
Leakage can be of particular concern with compressible fibrous gaskets, which generally are fabricated from sheets of material composed of fiber, filler, and a binder. Because of their fibrous nature and because apertures of the gasket typically are die-cut, the gasket edges surrounding the apertures tend to be somewhat porous. Since these porous edges usually are exposed to the fluid being sealed, intersticial leakage can be a particular problem with fibrous gaskets. Interfacial leakage can be caused by compression failure of the gasket material or by rough or warped flange surfaces. Thin flanges and poor bolt placement can result in regions of substantially reduced compression stress on a gasket, which also can lead to interfacial leakage.
In some instances, the sealability of a gasket can be enhanced by providing all of the surfaces of the gasket with a coating or by impregnating the gasket with a resin. Fibrous gaskets are particularly likely to have such treatments since, in many cases, the porous material of the gasket itself, although compression failure resistant, is subject to intersticial and interfacial leakage as a result of the failure mechanisms discussed above. U.S. Pat. No. 3,661,401 discloses a gasket having a coating that covers both the exposed gasket faces and the edges that surround and define various internal apertures of the gasket. U.S. Pat. No. 4,499,135 discloses a fibrous gasket that is impregnated with a silicone resin to improve its resistance to leakage of water-antifreeze mixtures. Similarly, U.S. Pat. No. 4,600,201 discloses a gasket impregnated with a polymerizable liquid impregnating agent to enhance sealability.
While coating and impregnation can improve the sealability of a gasket, unfortunately they inherently tend to degrade the compression failure resistance of the gasket. This is because, among other things, the coating and impregnating agents, which themselves exhibit good sealability but poor compression failure resistance, tend to penetrate and become a part of the gasket material. This reduces the gasket's overall compression failure resistance and thus reduces the ability of the gasket to function well under higher flange pressures where compression failure is more likely. As a result, coated and impregnated gaskets such as those disclosed in U.S. Pat. Nos. 3,661,401, 4,499,135 and 4,600,201 can perform poorly under high flange pressures, which severely limits the applications in which such gaskets can be used.
Other gaskets include special fillers to enhance their sealability. For example, U.S. Pat. No. 5,240,766 discloses a soft porous gasket sheet material formed from fiber, a binder, and a filler that provides enhanced sealability at higher temperatures. U.S. Pat. Nos. 5,536,565 and 5,437,767 also describe a gasket sheet material formed from fiber and a gel-forming mineral filler that provides the gasket with enhanced sealing properties, especially against polar liquids. While such fillers, like coatings and impregnations, can improve the sealability of gaskets, they also tend inherently to degrade the compression failure resistance of the gasket material and therefore reduce the ability of the gasket to withstand higher flange pressures. As a result, gaskets with specialized fillers to enhance sealability such as those disclosed in U.S. Pat. Nos. 5,240,766, 5,536,565 and 5,437,767 also can be severely limited in range of application.
It will thus be appreciated that for fibrous and perhaps other types of compressible gaskets, sealability and compression failure resistance have heretofore been mutually incompatible gasket properties. In other words, measures taken to enhance the sealability of such gaskets inherently tend to reduce compression failure resistance and vice versa. As a result, manufacturers of gaskets, and particularly fibrous gaskets, have engaged in proverbial balancing acts in order to design and produce gaskets with acceptable sealability and also acceptable compression failure resistance for a particular application. The problem, of course, is that each of these properties necessarily becomes a compromise and neither is optimized.
Another type of gasket used in many applications is known as a controlled compression rubber gasket. These types of gaskets incorporate molded rubber or polymer beads that are placed into a flanged joint in such a way that the amount of compression or compressive stress applied to the bead is predetermined and fixed by incompressible members. Such gaskets can take several forms. One form of a controlled compression rubber gasket is the common O-ring gasket, wherein a molded rubber bead is nested in a groove formed in the mating surface of one of a pair of flanges. The depth and width of the groove are carefully determined such that the compression stress on the rubber when the flanges are bolted together is known and thus controlled. In another form of controlled compression rubber gasket, a rubber bead or strip is molded onto the interior edge of a metal or plastic shim or carrier surrounding an interior aperture. The rubber bead is wider than the thickness of the shim and therefore can never be compressed to a thickness smaller then the thickness of the shim when the gasket is clamped between a pair of mating surfaces. Thus, the amount of compression applied to the rubber bead is limited by the thickness of the shim. In another example, a rubber bead is molded into grooves on one or both sides of a plastic carrier, which is disposed in a joint to be sealed. Metallic compression limiters, such as washers embedded in the carrier or shouldered bolts, provide a positive compression limit on the rubber and plastic of the gasket. Controlled compression rubber gaskets may also be found in the form of a rubber sheet or coating of a specific shape and profile molded onto both sides of a metal carrier with embedded washers or other means of compression limitation used to control the amount of compressive stress applied to the rubber coating.
U.S. Pat. No. 5,194,696 of Read illustrates one type of controlled compression rubber gasket wherein a rubber bead is molded onto the interior edge of a incompressible plastic carrier, the bead being wider than the thickness of the carrier. The gasket is placed between the mating flanges of a hard disc drive case and the flanges are bolted together until they engage the plastic carrier. The rubber bead is thus compressed between the flanges but never less than the thickness of the carrier such that the compressive stress applied to the bead is limited by the carrier thickness.
While the physical form of controlled compression gaskets varies, the sealing mechanism is common to all. Specifically, the beads of such gaskets are formed from a polymeric or rubber compound that is reasonably stable when in contact with heat and the particular fluid being contained. The spring rate of the compound in conjunction with the limited maximum compression stress provided by the carrier thickness or other compression limitation mechanism and the stiffness of the flanges yield a predetermined minimum and maximum surface stress between the rubber bead and the flange surfaces sufficient to prevent interfacial leakage. Spring rate of the bead is determined by the type and degree-of-cure of the rubber or polymer compound, the shape and contact area of the bead, and the thickness of the bead. The thickness of the compression limiter or depth of the groove in the case of O-ring seals is carefully designed to yield a compression stress on the bead that is sufficient to form a seal but not so high as to crush the bead. It will thus be seen that the performance of controlled compression gaskets is highly dependent upon the characteristics of the bead material and degree of compression provided by the compression limiting components. Too much compression can lead to crushing of the bead while too little can result in insufficient compression stress to establish a seal.
While controlled compression rubber gaskets have been used in many applications, they nevertheless suffer from a failure mechanism known as Compressive Stress Relaxation (CSR) failure in which the surface stress that prevents interfacial leakage diminishes over time. The CSR failure mechanism is a combination of several competing effects including, but not limited to, rearrangement of polymer molecule chains in response to the stress state, shrinkage of the bead due to molecular chain cross-linking, softening and swelling of the bead due to fluid penetration, and degradation of the polymer molecule chains due to heat, fluid, and oxygen exposure. Since the flange gap in which the bead resides is fixed by rigid compression limiters, these competing effects tend to reduce the compressive stress on the bead over time, which leads to leakage. Further, controlled compression gaskets tend to be substantially more expensive to manufacture than die-cut fibrous gaskets, which, among other factors, makes controlled compression gaskets an unacceptable alternative to fibrous gaskets in many applications.
A need therefore exists for an improved compressible fibrous gasket that retains the economy and wide application range of traditional fibrous gaskets and that also provides a superior and longer lasting seal. The properties of sealability and compression failure resistance should be de-coupled such that each can be optimized for a particular application without compromising the other. Such a gasket should exhibit excellent to complete sealability in a wide variety of joint types while at the same time having the highest possible resistance to compression failure where such failure is likely. The failure modes associated with controlled compression rubber gaskets should be successfully addressed, as should problems with warped or rough flange surfaces. A method of fabricating such a gasket that is economical, efficient, and reliable is also needed. It is to the provision of such a gasket and fabrication method that the present invention is primarily directed.